1. Field of the Invention
The present invention relates to checking materials and articles for defects by ultrasonic oscillations, and more particularly to an ultrasonic flaw detector for immersion testing of articles.
The invention may be used in devices intended for detecting defects in articles made from various materials. Most advantageously it can be used for non-destructive testing of thin-walled articles, for example, pipes.
2. Description of the Prior Art
Checking articles for defects by means of ultrasonic oscillations is widely known. It is also known that for generating ultrasonic oscillations in devices for detecting invisible defects use is made of the inverse piezoelectric effect (cf. USSR Inventors' Certificates, Nos. 82,273; 136,088; 179,076). These devices (flaw detectors) generally comprise piezoelectric elements having electrodes electrically connected with a source of high-frequency electric oscillations. Checking articles is effected, as a rule, by pulses, which allows the piezoelectric element to be used both for receiving ultrasonic waves reflecting from defects and for converting these waves into electric oscillations. The oscillations are analyzed with the aid of computing devices electrically connected with the piezoelectric element.
The detection efficiency of the above flaw detectors is improved by increasing the energy density of the ultrasonic oscillations applied to articles being checked for defects. To this end use is made of focusing elements permitting the sensitivity of flaw detectors to microflaws to be greatly enhanced. The ultrasonic oscillations are known to attenuate rapidly in gaseous environment and much slower in liquids. Therefore the ultrasonic flaw detectors are generally used for testing articles applying immersion method i.e. through a layer of liquid between the ultrasonic source and the article to be tested, thereby ensuring a good acoustic contact therebetween. Focusing the energy of the ultrasonic oscillations in the case of the immersion method is effected with the aid of focusing planoconcave lenses.
The construction of such flaw detectors allows application of the piezoelectric plates of a round or rectangular shape, made from quartz or piezoelectric materials, for example, lithium sulphate or barium titanate. In case a round piezoelectric plate is used the concave surface of the focusing lens is shaped spherical. In the case of a rectangular piezoelectric plate the concave surface of the focusing lens is shaped cylindrical.
The shape of the piezoelectric plate and hence of the focusing planoconcave lens is selected depending on what kind of defects are to be detected with a given flaw detector. For example, focusing planoconcave lenses with a spherical concave surface are used for detecting point defects (flaws and cavities) and defects being transverse relative to the generatrix of the pipe, whereas focusing planoconcave lenses with a cylindrical concave surface are used for detecting defects being longitudinal relative to the pipe generatrix. This is explained by the fact that the focusing planoconcave lenses with a spherical concave surface has a focal spot in the form of a point, whereas the lenses with a cylindrical concave surface has an elongated focal spot in the form of a line formed by plurality of focal points. Since articles, including pipes, may have defects being longitudinal or transverse relative to the pipe generatrix, as well as point defects, it is necessary for testing one and the same article to use in combination the focusing lenses with a spherical concave surface and lenses having a cylindrical concave surface, which complicates the construction of flaw detectors and their adjustment in the course of testing. The efficiency of the above method using focusing lenses of different shapes is relatively low because the focal spot area of the focusing lens having a spherical concave surface is small.
To use an ultrasonic flaw detector having a lens with a spherical concave surface capable of detecting transverse defects it is installed at an angle to the surface of the article being checked. For example, when used for checking pipes the flaw detector is positioned so that its acoustic line lies in a plane passing through the pipe axis at an angle relative to the generatrix of this pipe. This method is widely applied nowadays.
There are known ultrasonic flaw detectors using an immersion method for checking articles for defects as described above (cf., for example, "Pribory dlia nerasrushajustchego kontrolia materialov i izdelij, M., "Mashinostroenie", 1976 V.2, pp. 191-192, 179-183). These flaw detectors comprise a focusing planoconcave lens, a piezoelectric plate with electrodes applied onto the surface thereof and whereto are applied high frequency electric oscillations, and an ultrasonic oscillation attenuation unit, all said three elements contacting each other and being arranged in said order so that the main symmetry plane of the concave surface of the focusing planoconcave lens and that of the piezoelectric plate coincide.
However, when the above flaw detector is installed at an angle to the surface of the article being tested its focal spot deforms taking the shape of ellipse whose larger axis coincides with the directrix of the article. As a result focusing of the ultrasonic beam on the periphery of the focal spot in the direction of the larger axis of ellipse is distorted, which results in a lower sensitivity to defects in said regions of the focal spot. The lowest sensitivity of the detector is observed at the places which are more distant from the acoustic line of the detector.